In our Python API for accessing measurement hardware we allow the customer to retrieve their measurements in numpy arrays in case they want to do additional processing.

When using numpy 1.14.5 or earlier, we are able to load our C DLL with no issue and make the appropriate calls using ctypes.

When using numpy 1.15.0rc1, we get an error when ctypes tries to load the DLL saying "[WinError 126] The specified module could not be found".

I am using tox to run tests so it is using venv and all I have to do is change the version of numpy and it will start working. This means the DLL is in the path as expected and all dependencies are also loadable.

I have verified that numpy is the only installed package whose version is changing.

I have verified that the path environment variable has the appropriate directory even in the failure case.

I have verified that this happens on Python 2.7 and 3.6, both 32 bit and 64 bit.

As far as I know we are using ctypes correctly. At this point I don't have any more ideas on what to look at.